Five possible contenders for Trump's National Security Advisor

National Security Advisor Michael Flynn’s resignation came because his tenure had become “unsustainable,” said top aide Kellyanne Conway on NBC's "Today" show he was found to be “misleading” Vice President Mike Pence over his contacts with Russian officials. Flynn said that the information he provided about his contacts with Russia were "incomplete.”

President Trump tweeted afterward that the “real story here” is the comprised of the “illegal leaks coming out of Washington.”

The real story here is why are there so many illegal leaks coming out of Washington? Will these leaks be happening as I deal on N.Korea etc?

Retired Lt. Gen. Keith Kellogg is currently interim National Security Adviser. He served in Vietnam as officer and rose through the ranks. Kellogg also worked in the office of the Joint Chiefs of Staff, and on the homeland security team of Oracle, the software corporate giant.

Tom Bossert is a cybersecurity expert who once worked in the George W. Bush administration. He now serves as the Homeland Security Adviser. He co-authored a 2007 study that established the government’s security policies following the 9/11 terror attacks. Bossert defended the use of military force in Afghanistan and Iraq as “just,” despite Barack Obama’s objections.

Stephen J. Hadley served as the National Security Adviser to George W. Bush. He also served on several advisory boards, including defense firm Raytheon, and RAND's Center for Middle East Public Policy. He is a partner with former Secretary of State Condoleezza Rice and former Secretary of Defense Robert Gates in a consulting firm.

David Petraeus is a retired Army four-star general whose military career spanned 37 years. He served as commander of US Central Command and Director of the Central Intelligence Agency under Barack Obama until his resignation over an extramarital affair. He eventually pleaded guilty to mishandling of classified documents. He was briefly considered by Trump to serve as Secretary of State.

Robert S. Harward is a retired Vice Admiral and US Navy SEAL , and also former Deputy Commander of US Central Command, under General James Mattis, who is now the Secretary of Defense. He also served as the National Security Council Director of Strategy and Policy for the Office of Combating Terrorism, and is also the CEO for Lockheed Martin in the United Arab Emirates. Both the New York Times and Washington Post report that he is the leading contender to take over.

While the position is appointed by the president, and does not require a lengthy confirmation hearing from the Senate, it is likely that it will become the focus of Trump’s critics among Democrats..